=The Rhomboid Muscle= (Fig. 70, 21).--In order to make intelligible the position of the rhomboid in the superficial layer in quadrupeds, it appears to us necessary to recall the anatomical characters of the muscle as found in man. The rhomboid arises from the inferior portion of the posterior cervical ligament, from the spinous process of the seventh cervical vertebræ and the four or five upper dorsal; thence passing obliquely downwards and outwards, it is inserted into the spinal border of the scapula, into the portion of this border which is situated below the spine; it sometimes extends to the middle of the interval which separates this latter from the superior internal angle of the same bone. The portion of the muscle which arises from the cervical ligament and the seventh cervical vertebra is often separated from the lower portion by a cellular interspace. For this cause some anatomists have described the rhomboid as consisting of two parts--the superior or small rhomboid and the inferior or large rhomboid, on account of the position occupied by each, and of their difference in volume. This muscle can only be seen in the region of the back, in the space limited externally by the spinal border of the scapula, below by the latissimus dorsi, and internally by the trapezius, which covers it in the rest of its extent. It is not in this space that it is seen in certain quadrupeds. As we pointed out in the section on osteology, the spinal border of the scapula is short, and it seems to be due to this limitation in length that the trapezius and the latissimus dorsi muscle are, at this level, in contact the one with the other in such a way that they fill up the interval in which the rhomboid is seen in man. In the horse we can partly see it in the superficial muscular layer, but in the region of the neck only, at the superior border of the shoulder. Indeed, as we have already pointed out, the trapezius does not reach the occipital protuberance; for this reason a part of the anterior portion of the rhomboid may be seen--that is, the portion which corresponds to the superior part of the human muscle. But whether it be covered by the trapezius, or, as we find in the cat and dog, by the _mastoido-humeral muscle_ (see p. 150), which is very broad in this region, we do not the less recognise its presence; and in the horse and ox, in particular, it forms an elongated prominence beginning at the level of the scapula, and tapering as it ascends, towards the posterior part of the head. [Illustration: FIG. 70.--MYOLOGY OF THE HORSE: SUPERFICIAL LAYER OF MUSCLES.